The issue there then is that you need people willing to switch. A big issue in Splash Damage's previous game (Quake Wars: Enemy Territory) is that it allows players the option to forego actually advancing the game to play whatever class they personally enjoyed with no desire to deviate or actually win the map, tossing that responsibility off to others. I found myself forced as the only person wanting to actually advance to play a class I had no interest in because everyone else was busy screwing around. This is going to be the same, and it will likely die because of it.
Basically what I'm saying in this blurb: If you want the game to succeed, main Engineer. Nobody else is going to.
Ignoring the fact that there is no true favorite among the four classes, and each have their own devoted followings, Lets assume nobody ever played engineer by default. How hard is it, to spawn, then turn to your CP, and switch to an engineer? Not very, and any team trying to win, will do so very willingly, especially since the objective wheel will be telling them to do so.
Perhaps you should learn more of brink, before critizicing it? It seems you are unaware of the entire aspect of switching classes mid-game.